Hitherto, naphthaldehydes have been generally synthesized through known processes. Examples of such processes include a process for producing 7-methyl-1-naphthaldehyde, including reacting 2,7-dimethylnaphthalene with a halogenating agent, to form a monohalogenated species, and oxidizing the produced 7-methyl-2-halogenomethylnaphthalene (see Patent Document 1); a process for producing aromatic aldehydes, including reacting an aromatic halomethyl compound with nitric acid in the presence of a surfactant (see Patent Document 2); a process for producing dimethylnaphthaldehyde, including adding aluminum chloride to a mixture containing hydrogen chloride, zinc cyanide, and dimethylnaphthalene (see Non-Patent Document 1); and a production process including converting alkylnaphthalene to alkylnaphthaldehyde in the presence of hydrogen cyanide and aluminum chloride (see Non-Patent Document 2). Another known process for producing dialkylbenzaldehyde from dialkylbenzene includes reacting dialkylbenzene with carbon monoxide in the presence of a catalyst formed from hydrogen fluoride and boron trifluoride (see Patent Document 3).    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 08-268990    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 50-117737    Patent Document 3: U.S. Pat. No. 4,460,794    Non-Patent Document 1: F. M. Aslam and P. H. Gore, J. Chem. Soc., Perkin Trans. I, 1972, p. 892 and 893    Non-Patent Document 2: L. E. Hinkel, E. E. Ayling, and J. H. Beynon, J. Chem. Soc., 1936, p. 339 and 342